Brominated polystyrenes are well established as flame retardants for use in thermoplastics, e.g., polybutylene terephthalate, polyethylene terephthalate and polyamides. Recently, interest has been shown for expanding their use to syndiotactic polystyrene and polycyclohexylene dimethylene terephthalate. Generally, brominated polystyrenes are produced by a reaction between polystyrene and a brominating agent (e.g., bromine or bromine chloride) in the presence of a solvent (e.g., dichloroethane) and a Lewis acid catalyst. Within this broad context, the prior art has developed several processes which strive to obtain a low cost but high performing brominated polystyrene. Low cost is self-explanatory. Performance is predicated in part upon having a suitable bromine content (60-67 wt % generally being preferred), a solution color (.DELTA.E=20-35) and a chlorine content (the maximum being 1.5 wt %). The process chosen will determine the particular brominated polystyrene produced and thus, its qualities.
The bromine and chlorine content, and the color (it is believed) are properties of the structure of the particular brominated polystyrene being considered. The bromine content applies to the sum of (1) the bromine which is substituted onto the aromatic portions of the polymer, (2) the bromine which is substituted onto the alkyl portion of the polymer, e.g., the polymer backbone or which is present due to alkylation of the aromatic portion of the polymer, and (3) any ionic bromine present, e.g., sodium bromide. The undesired alkylation reaction which occurs during the bromination of polystyrene is catalyzed by the Lewis acid bromination catalyst used, and the reaction solvent (usually a 1-3 carbon atom dihaloalkane) serves as the alkylating agent. The bromine for (1) is referred to herein as aromatic bromide, while the bromine for (2) is referred to as alkyl bromide. Even though ionic bromine (hereinafter ionic bromide) can contribute to the bromine content, its contribution is almost always small to insignificant. Ionic bromide is not part of the polymer structure and is usually washed almost entirely from the brominated polymer product before the bromine content is measured.
The color of the brominated polystyrene is also believed to be due to polymer structure and not the result of some discreet impurity. Color may be caused by the above-mentioned alkyl bromide and/or the below-mentioned alkyl chloride substituents on the aromatic moieties.
The chlorine content is credited to chlorine which, like the bromine, is part of the polymer structure as an aromatic and/or an alkyl chloride. The use of bromine chloride as the brominating agent is the largest contributor to the chlorine content.
As a universal proposition, it is preferred that the brominated polystyrene have a minimized alkyl bromide and/or alkyl chloride, i e., alkyl halide, content. Alkyl halides are not desirable as they are not as thermally stable as are aromatic halides and, thus, they can be easily converted to hydrogen halide, e.g., HBr or HCl, under normal end-use processing conditions. Hydrogen halide, in the presence of moisture, can cause severe corroding of metal process equipment. Also, there is the matter of color, which is also believed to be impacted by some alkyl halides. A brominated polystyrene having almost all aromatic bromide (ar-bromine) will have desirable flame retarding characteristics as the bromine will not leave the aromatic moiety at processing temperatures, but rather, will leave at the very high temperatures which are encountered in the vicinity of an approaching flame front.
Outside of whether or not the halide is present as an aromatic or an alkyl halide, it is also desirable to minimize the total chlorine content of the brominated polystyrene as chlorine is not as efficacious or as stable a flame retardant constituent as is bromine.
The desirability of obtaining a high aromatic bromine content along with a low alkyl halide and total chlorine content is, unfortunately, not matched by the ability of prior art processes to produce same. Even though the art has proffered many processes which are claimed to produce a superior brominated polystyrene, none have actually been shown to deliver on their promise. See U.S. Pat. No. 4,200,703; U.S. Pat. No. 4,352,909; U.S. Pat. No. 4,975,496 and U.S. Pat. No. 5,532,322. A review of the Examples in these patents, which are reported to be actual experiments, shows that a high bromine content, say 68 wt % or above, is not obtained.
Further, the prior art brominated polystyrenes do not exhibit high thermal stability. Prior art polymers exhibit a 1% weight loss at temperatures less than 336.degree. C. when submitted to Thermo-gravimetric Analysis (TGA) and, indeed, most exhibit a 1% weight loss at temperatures around 300.degree. C. A low thermal stability is not desired when the brominated polystyrene is formulated with thermoplastic formulations which will be exposed to high processing temperatures.
Additionally, it has been demonstrated that prior art processes for the manufacture of brominated polystyrene give rise to significant cleavage of the polymer chain. This cleavage results in the produced brominated polystyrene having an M.sub.w, as measured by Gel Permeation Chromatography, which is significantly lower than the calculated theoretical M.sub.w of the brominated polystyrene. The calculation is based upon the bromine content (wt %) of the brominated polystyrene product and the M.sub.w of the polystyrene reactant at reaction initiation. It is advantageous if the theoretical M.sub.w and actual M.sub.w of the produced brominated polystyrene are close, given the .+-.margins of error for GPC, since such closeness evidences a paucity of polymer cleavage. The degree of cleavage should be minimized since cleavage results in an increase of alkyl end groups in the brominated polystyrene, which alkyl end groups provide loci for the facile formation of the undesirable alkyl halides discussed above.
Commonly-owned co-pending application Ser. No. 08/852,462 and its parent application Ser. No. 08/727,341, filed Sep. 26, 1996, now U.S. Pat. No. 5,677,390, both referred to at the outset, provide and describe, inter alia, novel thermally stable brominated polystyrene, and novel process technology for producing such brominated polystyrenes.